Traditional MIMO systems are mostly combined with the multi-carrier orthogonal frequency division multiplexing (OFDM) technology. However, the OFDM has the problem of higher peak-to-average ratio and sensitivity to frequency offset and phase noise. In contrary, the single carrier (SC) technology lowers the peak-to-average ratio and the sensitivity to frequency offset and phase noise, therefore, the MIMO system combined with SC also attracts extensive attention. In the single-carrier MIMO system, though not so sensitive as the OFDM system, the influence of sampling frequency offset and phase noise on the performance of a system is still non-ignorable. A common solution is to insert pilot frequencies into data for frequency offset estimation and phase tracking.
The existing pilot design solutions are divided into two types. The first type is time domain pilot, which means that known time domain sequences are inserted into data for frequency offset estimation and phase correction. In a typical time domain pilot design solution of a single-carrier system, a unique word (UW) is inserted into data, and the UW is utilized for synchronization and also for substituting a cyclic prefix (CP) to resist against the multipath effect. However, the MIMO system often needs a cyclic shift operation, which means that the directly inserted UW cannot replace the CP to resist against multipath, and thus may also cause interference between symbols. Therefore, the traditional UW time domain pilot design solution is not suitable for the single-carrier MIMO system. In another commonly used single-carrier pilot solution, pilot design is conducted in a frequency domain, which however generally requires the addition of modules for FFT and IFFT operations in a transmitting terminal. Therefore, compared with time domain pilot, the pilot of frequency domain may increase the complexity of the transmitting terminal of a system. In addition, the time domain pilot design of a single-carrier MIMO system also has the problem of mutual interference of pilot frequencies between different antennas.